The need for bone implants is increasing due to accidents, genetic defects, osteoporosis and cancer. This need is expected to expand as the number of older people in the population increases dramatically in the next two decades. In addition, new therapies are allowing patients with cancer to live longer.
Intramedullar nails or rods are implantable devices that are inserted into the medullary canal of long bones to provide support. A typical intramedullary nail comprises a rod. At the head of the rod there is a hole transverse to the longitudinal axis of the rod. A means for anchoring the rod, such as a screw is passed through the hole to fasten the rod to the bone. The nails or rods are placed into the medullary canal through a hole that is drilled at one end of the bone.
There have been many attempts to provide improved intramedullary nails. For example, U.S. Pat. No. 6,443,954 discloses an intramedullary system for securing portions of a bone together has a lag screw assembly extending through a radial bore in an intramedullary nail. The lag screw is inserted into one portion of a bone and deployed to fix the leading end. The intramedullary nail is placed in the intramedullary canal of a portion of the bone and the trailing end of the lag screw assembly is adjustably fixed in the radial bore to provide compression between the lag screw assembly and the intramedullary nail. The intramedullary nail has a cap screw in the proximal end holding the lag screw assembly and a tang in the distal end. The tang has legs extending through the nail to fix the distal end in the intramedullary canal.
U.S. Pat. No. 6,527,775 discloses methods and devices for treating fractures in or adjacent the wrist and distal forearm employ an intramedullary interlocking fixation rod (i.e., it interlocks the distal and proximal fracture fragments together) to stabilize the skeletal structure in a manner which can inhibit the amount of collapse or loss in skeletal length exhibited by a patient with a distal radius fracture.
U.S. Pat. No. 6,755,862 describes an intramedullary strut in the form of nested telescopic members. The strut can be telescopically extended into the medullary canal to provide support.
An ideal permanent bone implant would be compatible with living tissue, easy to insert and be able to withstand the stresses typically placed upon bones during normal moment. For cancer patients, it is also desirable to have an implant that prevents the build-up of pressure in the medullary canal that can promote the dissemination of metastatic cells. It would also be desirable to have an implant that bears the full load to which the bone is normally subjected. However, it has proven difficult to develop an implant having all these features.